1. Field of the Invention
The present invention relates to an electronic control device for an internal combustion engine, particularly to a purge control which supplies evaporated fuel generated in a fuel tank to an engine.
2. Discussion of Background
FIG. 6 is a construction diagram of a conventional air-fuel ratio control device of an engine disclosed, for instance, in Japanese Unexamined Patent Publication No. 255559/1988. In FIG. 6, a throttle valve 16, a surge tank 17 and an injector 3 are successively installed at an intake air passage 2 which supplies intake air to a combustion chamber 15 of an engine 1. An evaporated fuel discharge passage 6 is connected to the intake air passage 2 downstream from the throttle valve 16. The upstream end of the evaporated fuel discharge passage 6 is connected to a canister 18 of an evaporated fuel discharge restraining device 5 through a control valve 7 which is driven by a duty solenoid valve. The canister 18 incorporates an adsorbent which adsorbs the evaporated fuel. The evaporated fuel from a fuel tank 19 is supplied to the intake air passage 2 through the evaporated fuel discharge passage 6 when the control valve 7 is operated to open, in accordance with an opening degree of the control valve.
An air-fuel ratio sensor 21 is installed at an exhaust passage 8 which is an air-fuel ratio detecting means. Detecting signals of the air-fuel ratio sensor 21 are outputted to a control unit 22. Fuel injection pulses are outputted to the injector 3 based on a feedback control whereby the detected air-fuel ratio conforms to a target air-fuel ratio in accordance with the output of the detecting signal. A duty control signal is outputted from the control unit 22 to the control valve 7 whereby the opening degree, that is, a supply quantity of the evaporated fuel is controlled.
The control unit 22 is respectively inputted with a rotation signal of the engine from a rotation sensor 23, an intake air quantity signal from an intake air quantity sensor 24 and a throttle signal from a throttle sensor 25 which detects the opening degree of the throttle valve 16, for detecting a running condition of the engine. Further, the control unit 22 fundamentally calculates basic fuel injection pulses from the intake air quantity and the rotation number of the engine and calculates final injection pulses by correcting the basic fuel injection pulses by various conditions such as the output of the air-fuel sensor 21 thereby forming an output to the injector 3. A flow chart is shown in FIG. 7 which shows these controls.
Further, a duty signal is determined from a map which has been predetermined in accordance with the running condition of the engine 1, and the duty signal is outputted to the control valve 7. A dulling treatment is performed wherein the duty signal gradually increases when a supply quantity of the evaporated fuel is increasing. On the other hand, the duty signal is controlled to decrease without performing the dulling treatment when the supply quantity of the evaporated fuel is decreasing. Normally, the supply quantity of the evaporated fuel is reduced in a deceleration period. In the deceleration period, fuel adhered to the intake air passage 2 is also supplied to the combustion chamber 15. Therefore, when the evaporated fuel is gradually reduced, large amounts of the evaporated fuel and the adhered fuel are supplied to the combustion chamber 15, whereby the air-fuel ratio is considerably deviated. To prevent the above phenomena, the dulling treatment is not performed when the supply quantity of the evaporated fuel is decreasing.
Further, there is a case disclosed in, for instance, Japanese Unexamined Patent Publication No. 45442/1988, wherein the operation copes with the considerable deviation of the air-fuel ratio due to the execution of the purge control by expanding a range of the air-fuel ratio control in which an air-fuel ratio correction coefficient of the air-fuel ratio control can be provided, when the purge control is performed.
The former conventional air-fuel ratio control device of the engine has been constructed as above, wherein the dulling treatment is performed such that the supply quantity of the evaporated fuel gradually increases. However, the control is performed by a constant amount of dulling treatment irrespective of the concentration of fuel vapor. Therefore, when the concentration of the fuel vapor is large, the influence thereof on the air-fuel ratio is considerable, which significantly deteriorates the exhaust gas. Further, when the concentration of the fuel vapor is small, the purge control can not sufficiently be performed. Further, in the other conventional device, the control range of the air-fuel ratio control is always expanded when the purge control is being performed. The control range is expanded even when the expansion of the control range is not necessary, which gives rise to a possibility of an erroneous operation by noise or the like, whereby the operation becomes unstable.